At least seven people were killed and 25 were injured Monday after a “superfog” of smoke from marsh fires and dense fog descended on Interstate 55 in Louisiana, causing a massive pileup that left over 100 vehicles smashed and in flames.

An estimated 158 vehicles were involved in the many crashes near New Orleans, according to a preliminary investigation by the Louisiana State Police. Officials warned the death toll could climb as the search for victims continue. At least 25 people were taken to local hospitals, while others with less severe injuries sought medical care on their own, a Facebook post by the Louisiana State Police said.

Videos and pictures taken by drivers, passengers and responding law enforcement show crushed vehicles packed among scattered debris along the interstate. Some cars were overturned and atop of others. The elevated interstate, which is expected to be closed for the foreseeable future, passes over swamp and open water between lakes Pontchartrain and Maurepas.

Shortly after the series of crashes began, a portion of the area "caught on fire," according to Louisiana State Police. One tanker truck "carrying a hazardous liquid" was "compromised" and had to be off-loaded, delaying first responders from assessing vehicles and searching for people potentially trapped inside, police said.

State troopers said the removal of cars, trucks and tractor-trailers will take additional time. Following the clean-up of the interstate, the Louisiana Department of Transportation and Development will inspect the bridge for any deficiencies.

Louisiana Gov. John Bel Edwards released a statement on X Monday asking for prayers for those hurt and killed in the pile-up, as well as for their families. He urged locals to donate blood to "help replenish supplies that are being drained today to care for the wounded."

What is 'superfog'?

Superfog is the combination of smoke and fog, Phil Grigsby, a meteorologist with the National Weather Service, told USA TODAY.

On Monday, dense morning fog began to blanket the area around the interstate at around 3 a.m. Some hours later, wind direction changed and carried heavy smoke from a marsh fire near the Bayou Sauvage Urban National Wildlife Refuge toward I-55, Grigsby said. The combination of the two caused zero visibility and the closure of a nearby bridge. The fire has been burning for at least nine days.

“Superfog takes the fog that you normally would have, and makes it even more dense as that smoke gets mixed in and reduces the visibility until you basically have near zero visibility develop," Grigsby said.

He added that superfog events are not common, but they do occasionally happen – especially when there’s a wildfire burning in the area.

The marsh fire is on private land, and NWS does not know what caused the blaze.

Contributing: Saman Shafiq, USA TODAY; The Associated Press